The present invention relates to a method of exchanging the displacement element of a refrigerator having a divided housing whose parts can be separated from one another in such a manner that the displacement element becomes accessible. Additionally, the invention relates to a refrigerator designed for implementation of the method.
Refrigerators of this type are low-temperature cooling machines in which thermodynamic circulation processes take place. A single-stage refrigerator essentially comprises a work chamber including a displacement element. The work chamber is connected in a certain manner alternatingly with a high pressure and a low pressure gas source so that the thermodynamic circulation process takes place during the enforced back and forth movement of the displacement element. This causes the operating gas to be conducted in a closed circuit. The result is that heat is extracted from a certain region of the work chamber. In two-stage refrigerators of this type and with helium as the working gas, temperatures down to less than 10.degree. K. can be produced.
For repair and maintenance work there often arises the need to exchange the displacement element. This can be done in such a manner that the refrigerator is shut down and the cold regions of the refrigerator are permitted to warm up to room temperature. Without warm-up, condensible gases would precipitate at the cold surfaces and would endanger the renewed operation of the refrigerator. Since the temperatures at the cold locations of a refrigerator operated with helium are only a few degrees K, the presence of almost all gases normally occurring in the atmosphere are undesirable. After sufficient heating, the housing is opened and the displacement element is exchanged. After closing and thorough rinsing of the housing with the working gas, the refrigerator is put back into use.
A change of displacement elements effected in this manner takes a very long time since the warm-up periods and the restarting periods are added to the actual work times. Moreover, operation of the device or an instrument cooled by the refrigerator is interrupted during these times. For instruments filled with a large amount of liquid helium (100 liters and more) warming the refrigerator merely for an exchange of displacement elements is unfeasible for economic reasons. It requires removal of the liquid helium.
To avoid long interruptions of operation, it is known to effect the change of displacement elements with the aid of a glove box. The use of a glove box makes it possible to effect the exchange of displacement elements in a protective gas atmosphere. In this way, for example, condensible gases can be prevented from penetrating into the work or cylinder chamber of the displacement element and condensing at regions that are still cold. The use of the glove box has the advantage that the exchange of displacement elements can be effected at even lower temperatures, i.e., it is not necessary to wait for the cold regions of the refrigerator and the connected apparatus or instrument to warm up.
However, the use of a relatively expensive glove box is possible only where sufficient space is available. Moreover, the duration of the exchange work is still rather long (at least two hours). The reason for this is, firstly, the need for several rinsing processes in order to produce a sufficiently pure protective gas atmosphere within the glove box and these rinsing processes require the consumption of a relatively large amount of protective gas. Another reason is the difficulty of manipulation by means of the gloves in the glove box. Additionally, there exists the danger that the cold side of the displacement element to be replaced, once it has been pulled out, may touch the walls or gloves of the glove box. This would result in destruction of these parts, which are usually made of thin plastic sheets, and thus in contamination of the protective gas atmosphere. The increases in temperature occurring during the exchange work are not negligible, so that start-up times must still be added to the times for the direct exchange work. Finally, the costs connected with the high consumption of protective gas are quite considerable if helium must be employed.